runners: openocd do not try to iterate over gdb_init if None
runners: openocd do not try to iterate over gdb_init if None

Commits 49bcc08033c6d68bf846e47e91d9c7ffaa67cb7e introduced
a possible None object 'gdb_init', do not try to iterate over
this variable if it is a None object.

Signed-off-by: Julien Massot <ad45d338e0f9dc7767e692227f7b2fcd41b31818@iot.bzh>

galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr